For many people, reporting income is pretty straightforward. But for those who earn money a variety of different sources, it can be easy to forget a stray account.

Giovetti says some clients forget about small brokerage accounts they have, and since the IRS receives information from brokerage firms directly as well, there's a good chance you'll be contacted if your records don't match what the IRS receives. Because investment firms aren't required to submit documentation for their clients until the end of the February, it's often a good idea to wait until the beginning of March to file your return to make sure the reporting lines up.

If you worked side jobs and earned more than $600 at any one of them in a year, those employers should send you a Form 1099 so you can report that income on your taxes as well.

"If you left something off your return, you can be pretty sure the IRS will find out about it," said Connolly.